Simple block space time transmit diversity using multiple spreading codes

ABSTRACT

The present invention is an apparatus for a base station to transmit a data field of symbols. A first data field of symbols is generated and encoded to produce a second data field having complex conjugates of the symbols of the data field. The first and second data fields are then spread, wherein the first data field is spread using a first channelization code and the second data field is spread using a second channelization code. Each of the channelization codes are uniquely associated with one of a first and second antenna. An RF signal including the first and second spread data fields is then transmitted over a first and second antenna.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No.09/999,287, filed on Nov. 15, 2001; which claims priority fromProvisional Application No. 60/254,013, filed on Dec. 7, 2000.

BACKGROUND

[0002] The present invention relates to communications systems imploringcode division multiple access (CDMA) techniques. More particularly, thepresent invention relates to a transmission diversity scheme which canbe applied to a CDMA communication system.

[0003] Spacial diversity has been proposed for support of very high datarate users within third generation wide band code division multipleaccess systems such as CDMA. Using multiple antennas, the systemsachieve better gains and link quality, which results in increased systemcapacity. Classically, diversity has been exploited through the use ofeither beam steering or through diversity combining.

[0004] More recently, it has been realized that coordinated use ofdiversity can be achieved through the use of space-time codes. Suchsystems can theoretically increase capacity by up to a factor equalingthe number of transmit and receive antennas in the array. Space-timeblock codes operate on a block of input symbols producing a matrixoutput over antennas and time.

[0005] In the past, space-time transmit diversity systems havetransmitted consecutive symbols simultaneously with their complexconjugates. This type of system, though may result in symbol overlap atthe receiving end, with the amount of overlap being dependent on thelength of the impulse response of the propagation channel. In timedivision duplex (TDD) mode, this symbol overlap will have to beaccounted for in the joint detection receiver. The joint detector willhave to estimate the transmitted symbols and their conjugates, resultingin an increase in complexity of the joint detection.

[0006] In order to alleviate this increase in joint detection, systemshave been created which transmit two similar but different data fields.The first data field, having a first portion, D₁, and a second portion,D₂, is transmitted by the first antenna. A second data field is producedby modifying the first data field. The negation of the conjugate of D₂,-D₂*, is the first portion of the second data field and the conjugate ofD₁, D₁*, is the second portion. The second data field is simultaneouslytransmitted by the second antenna. This type of system results in thejoint detection implemented at the receiver needing only to estimate thesame amount of symbols as in the case of a single transmit antenna. Ablock diagram of this system is illustrated in FIG. 1.

[0007] Although the above system reduces the complexity of jointdetection for a single data block, joint detection requires the use oftwo joint detectors at the receiver in a system employing two transmitdiversity antennas. Each joint detection device estimates the data fromone of the antennas. The estimated data is combined to produce theoriginal data. Therefore, the receiver in such a system has a highcomplexity resulting in higher receiver expense.

[0008] Accordingly, there exists a need for a transmit diversity systemrequiring less complexity and receiver expense.

SUMMARY

[0009] The present invention is an apparatus for a base station totransmit a data field of symbols. A first data field of symbols isgenerated and encoded to produce a second data field having complexconjugates of the symbols of the data field. The first and second datafields are then spread, wherein the first data field is spread using afirst channelization code and the second data field is spread using asecond channelization code. Each of the channelization codes areuniquely associated with one of a first and second antenna. An RF signalincluding the first and second spread data fields is then transmittedover a first and second antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a block diagram of a prior art communication systememploying space-time transmit diversity.

[0011]FIG. 2 is a block diagram of a transmitter and receiver in acommunication system in accordance with the preferred embodiment of thepresent invention.

[0012]FIG. 3 is a flow diagram of the transmit diversity system of thepresent invention.

[0013]FIG. 4 is a graph of the performance of the transmit diversitysystem of the present invention.

[0014]FIG. 5 is a block diagram of a transmitter and receiver in acommunication system in accordance with an alternative embodiment of thepresent invention.

[0015]FIG. 6 is a flow diagram of an alternative transmit diversitysystem of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016]FIG. 2 is a block diagram of a transmitter 10, preferably locatedat a base station, and a receiver 20, preferably located at a userequipment (UE), in a CDMA communication system in accordance with thepreferred embodiment of the present invention. Although it is preferableto have the transmitter located at a base station and the receiverlocated at the UE, the receiver and transmitter may switch locations andthe present invention operate on an uplink communication. Thetransmitter 10 comprises a block encoder 11, a plurality ofchannelization devices 8, 9, a plurality of spreading sequence insertiondevices 12, 13, and a plurality of antennas 15, 16. Although FIG. 1illustrates a transmitter comprising two (2) antennas, it should beapparent to those having skill in the art that more than two (2)antennas may be used, such as N antennas.

[0017] A typical communication burst has two data fields separated by amidamble sequence. Preferably, the same encoding procedure, as discussedin the following, for one data field is also performed on the other datafield. Data to be transmitted by the transmitter 10 is produced by adata generator (not shown). The resulting data symbols (S₁, S₂, . . .S_(N/2)), (S_(N/2+1), S_(N/2+2), . . . , S_(N)) of the first data field,which can be represented by sub-data fields D₁ and D₂, are input intothe block encoder 11, preferably a block space-time transmit diversity(BSTTD) encoder. The block encoder 11 encodes the input symbols andgenerates the complex conjugate of D₁ and the negation of the conjugateof D₂: D₁*, -D₂*. The encoder 11 also changes the order of the symbolsso that -D₂* is ahead of D₁*. Preferably, an analogous encoding of thesecond data field is also performed.

[0018] In accordance with the preferred embodiment of the presentinvention, the data fields, D₁, D₂ and -D₂*, D₁* are forwarded to afirst and second channelization device 8, 9, respectively. The firstchannelization device 8 spreads the data blocks D₁, D₂ by a firstchannelization code, and -D₂*, D₁* by the second channelization device 9using a second different channelization code. Each of the spread datablocks from the first and second channelization devices 8, 9 are thenscrambled by the scrambling code associated with the transmitter 10.

[0019] Once the symbols D₁, D₂, -D₂*, D₁* have been scrambled, they aremixed with a first and second midamble through training sequenceinsertion devices 12, 13, producing two communication bursts 17, 18. Thetwo bursts 17, 18 are modulated and simultaneously transmitted to thereceiver 20 over antenna 15 and diversity antenna 16, respectively.

[0020] The receiver 20 comprises a joint detection device (JD) 24, aBSTTD decoder 22, a channel estimation device 23 and an antenna 26. Theantenna 26 of the UE receives various RF signals including thecommunication bursts 17, 18 from the transmitter 10. The RF signals arethen demodulated to produce a baseband signal.

[0021] The baseband signal is then forwarded to the joint detectiondevice 24 and the channel estimation device 23. As those skilled in theart know, the channel estimation device 23 provides channel information,such as channel impulse responses, to the joint detection device 24.

[0022] The joint detection device 24, coupled to the channel estimationdevice 23 and BSTTD decoder 22, utilizes the channel information and thechannelization codes to detect the soft data symbols d₁, d₂, -d₂*, d₁*in the received signal. The channel impulse response for each burst isdetermined using that burst's midamble sequence. Since each burst wastransmitted using a different spreading code, the joint detection device24 treats each burst as being transmitted by a different user. As aresult, any joint detection device which can recover data from differenttransmitter sites may be used. Such joint detection devices include zeroforcing block linear equalizers, detection devices using Cholesky orapproximate Cholesky decomposition, as well as many others. The jointdetection device 24 estimates the data symbols of each of the bursts 17,18 output by the transmitter antennas 15, 16 and forwards the estimatesto the BSTTD decoder 22.

[0023] The BSTTD decoder 22, coupled to the joint detection device 24,receives the estimated soft data symbols d₁, d₂ and -d₂*, d₁*corresponding to the antennas 15, 16 and decodes the symbols to yield asingle data field's soft symbols, d_(STTD).

[0024] The flow diagram of the present invention is illustrated in FIG.3. A data generator generates data to be transmitted to the receiver 20(step 301). Each data field is separated into two sub-data fields D₁, D₂(step 302). The sub-data fields D₁, D₂ are forwarded to the blockencoder 11 and the first channelization device 8 (step 303). Thesub-data fields forwarded to the block encoder 11 are encoded (step 304)and forwarded to the second channelization device 9 (step 305). Eachchannelization device 8, 9 spreads their respective data input using aseparate channelization code associated with a respective antenna 15, 16(step 306). The two spread signals are then scrambled, using thescrambling code associated with the base station (step 307) andtransmitted to the receiver 20 over diversity antennas 15, 16 (step308).

[0025] The receiver 20 receives a RF communication signal including thetwo spread signals from the diversity antennas 15, 16 (step 309),demodulates the signal and forwards the demodulated signal to thechannel estimation device 23 and joint detection device 24 (step 310).The received signal is processed by the channel estimation device 23(step 311) and the channel information applied by the joint detectiondevice 24 along with the channelization codes, to estimate the transmitsymbols from the diversity antennas 15, 16 (step 312). The detectedsub-data fields, corresponding to each diversity antenna 15, 16, areforwarded to the BSTTD decoder (step 313), which decodes the soft symbolsub-fields to yield a single data field's soft symbols, d_(STTD) (step314).

[0026] Similar to the preferred embodiment disclosed above, FIG. 5 is ablock diagram of an alternative transmitter 40, preferably located at abase station, and a receiver 50, preferably located a user equipment(UE) in a communication system. The transmitter 40 comprises a pluralityof channelization devices 48, 49, a plurality of spreading sequenceinsertion devices 42, 43, and a plurality of antennas 45, 46.

[0027] Data to be transmitted by the transmitter 40 is produced by adata generator (not shown). The resulting data symbols (S₁, S₂, . . .S_(N/2)), (S_(N/2)+1, S_(N/2)+2, . . . , S_(N)) of the first data field,which can be represented by sub-data fields D₁ and D₂, are input to afirst and second channelization device 48, 49, respectively. The firstchannelization device 8 spreads the data blocks D₁, D₂ by a firstchannelization code, and the second channelization device 49 spreads thedata blocks D₁, D₂ by a second different channelization code. Each ofthe spread data blocks from the first and second channelization devices48, 49 are scrambled by the scrambling code associated with thetransmitter 40.

[0028] Once the symbols have been scrambled, they are mixed with a firstand second midamble through training sequence insertion devices 42, 43,producing two communication bursts 44, 45. The two bursts 44, 45 aremodulated and simultaneously transmitted to the receiver 50 over antenna46 and diversity antenna 47, respectively.

[0029] The receiver 50 comprises a joint detection device (JD) 54, adecoder 22, a channel estimation device 53 and an antenna 51. Theantenna 51 of the UE receives various RF signals including thecommunication bursts 44, 45 from the transmitter 40. The RF signals arethen demodulated to produce a baseband signal.

[0030] The baseband signal is then forwarded to the joint detectiondevice 54 and the channel estimation device 53. The joint detectiondevice 54, coupled to the channel estimation device 53 and decoder 52,utilizes the channel information and the channelization codes to detectthe soft data symbols d₁, d₂, in the received signal. The channelimpulse response for each burst is determined using that burst'smidamble sequence. Since each burst was transmitted using a differentspreading code, the joint detection device 54 treats each burst as beingtransmitted by a different user. The joint detection device 54 estimatesthe data symbols of each of the signals 44, 45 output by the transmitterantennas 46, 47 and forwards the estimates to the decoder 52.

[0031] The decoder 52, coupled to the joint detection device 54,receives the estimated soft data symbols d₁, d₂ corresponding to theantennas 46, 47 and decodes the symbols to yield a single data field'ssoft symbols, d.

[0032] The flow diagram of the alternative embodiment is illustrated inFIG. 6. A data generator generates data to be transmitted to thereceiver 40 (step 601). Each data field is separated into two sub-datafields D₁, D₂ (step 602). The sub-data fields D₁, D₂ are forwarded tothe first channelization device 48 and to the second channelizationdevice 49 (step 603). Each channelization device 48, 49 spreads theirrespective data input using a separate channelization code associatedwith each antenna 46, 47 (step 604). The two spread signals are thenscrambled, using the scrambling code associated with the base station(step 605) and transmitted to the receiver 50 over diversity antennas46, 47 (step 606).

[0033] The receiver 50 receives a RF communication signal including thetwo spread signals from the diversity antennas 46, 47 (step 607),demodulates the signal and forwards the demodulated signal to thechannel estimation device 53 and joint detection device 54 (step 608).The received signal is processed by the channel estimation device 53(step 609) and the channel information applied by the joint detectiondevice 54 along with the channelization codes, to estimate the transmitsymbols from the diversity antennas 46, 47 (step 610). The detectedsub-data fields, corresponding to each diversity antenna 46, 47, areforwarded to the decoder 52 (step 611), which decodes the soft symbolsub-fields to yield a single data field's soft symbols, d_(STTD) (step612).

[0034] By using additional channelization codes, the above approachescan be applied to an antenna array having any number of antennas. Eachantenna has its own associated channelization code and midamble. If ablock encoder is used, the data field transmitted by each of theantennas has a unique encoding, allowing the use of a single jointdetector at the receiver.

[0035] The BSTTD transmitter with two channelization codes of thepresent invention allows for the use of a cheaper and simpler method oftransmit diversity. The use of different channelization codes pertransmit antenna requires only one joint detection device at thereceiver resulting in a less complex receiver system than those of theprior art. FIG. 4 is a graph showing the raw BER of various block STTDdecoders. The model is based on all the receivers using a block linearequalizer (BLE) based approach to JD. NTD means the single antenna case,i.e., no transmit diversity. STTD with 1 code is the traditional blockSTTD JD. STTD with 2 code is the disclosed block STTD transmitter.Simple STTD with 2 code is the transmission system disclosed in thealternative embodiment. As illustrated, the benefit of 2 codes for STTDcan be summarized as follows: 1) there is up to a 0.5 dB gain at 0.01raw Bit error rate over 1 code STTD; and 2) by eliminating the encodingblock in simple STTD with 2 code, the performance degradation is only0.2 dB at 0.1 raw BER and no degradation at 0.01 raw BER. Theperformance improvement over NTD is still 1.0 dB and 2.7 dB at 0.1 and0.01 raw BER.

What is claimed is:
 1. A base station including a transmitter fortransmitting a data field of symbols, the transmitter comprising: afirst and second antenna for transmitting said data field of symbols; anencoder for encoding said data field producing a second data fieldhaving complex conjugates of the symbols of said data field; and a firstand second spreading device for spreading said first and second datafields, wherein said first spreading device spreads said first datafield using a first channelization code and said second spreading devicespreads said second data field using a second channelization code, eachchannelization code being uniquely associated with one of said first andsecond antennas.
 2. The base station of claim 1 wherein said transmitterfurther comprises a first and second scrambling device for scramblingsaid first and second spread data fields by a single scrambling codeassociated with said transmitter.
 3. The base station of claim 2 whereinthe symbols of said first data field of symbols are grouped into a firstand second sub-data field.
 4. The base station of claim 3, wherein thesymbols of said second data field of symbols are grouped into a thirdand fourth sub-data field, said third sub-data field being the negativecomplex conjugate of said second sub-data field and said fourth sub-datafield being the complex conjugate of said first sub-data field.
 5. Abase station comprising a transmitter including: a first and secondmeans for transmitting a data field of symbols including a first datafield; a means for encoding said data field producing a second datafield having complex conjugates of the symbols of said data field; and afirst and second spreading means for spreading said first and seconddata fields, wherein said first spreading means spreads said first datafield using a first channelization code and said second spreading meansspreads said second data field using a second channelization code, eachchannelization code being uniquely associated with one of said first andsecond transmitting means.
 6. The base station of claim 5 wherein saidtransmitter further comprises a means for scrambling said first andsecond spread data fields by a scrambling code associated with saidtransmitting means.
 7. The base station of claim 6 wherein the symbolsof said first data field of symbols are grouped into a first and secondsub-data field.
 8. The base station of claim 7, wherein the symbols ofsaid second data field of symbols are grouped into a third and fourthsub-data field, wherein said third sub-data field is the negativecomplex conjugate of said second sub-data field and said fourth sub-datafield is the complex conjugate of said first sub-data field.
 9. A basestation including a transmitter for transmitting a data field ofsymbols, the transmitter comprising: a first and second antenna fortransmitting said data field of symbols; and a first and secondspreading device for spreading said data field, wherein said firstspreading device spreads said data field using a first channelizationcode, producing a first spread data field, and said second spreadingdevice spreads said data field using a second channelization code,producing a second spread data field, each channelization code beinguniquely associated with one of said first and second antennas.
 10. Thebase station of claim 9 further comprising a first and second scramblingdevice for scrambling said first and second spread data fields by asingle scrambling code associated with said transmitter.
 11. A basestation including a transmitter comprising: a first and second means fortransmitting a data field of symbols; and a first and second spreadingmeans for spreading said data field, wherein said first spreading meansspreads said data field using a first channelization code producing afirst spread data field and said second spreading means spreads saidsecond data field using a second channelization code producing a secondspread data field, each channelization code being uniquely associatedwith one of said first and second transmitting means.
 12. The basestation of claim 11 further comprising a means for scrambling said firstand second spread data fields by a scrambling code associated with saidtransmitting means.